Citrate Synthase: Difference between revisions
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'''Mechanism:''' The reaction mechanism for citrate synthase was proposed by James Remington. In this mechanism, three ionizable side chains in the | '''Mechanism:''' The reaction mechanism for citrate synthase was proposed by James Remington. In this mechanism, three ionizable side chains in the | ||
<scene name='Daniel_Eddelman_Sandbox_2/Cts_active_site/4'>active site</scene> of citrate synthase participate in acid-base catalysis: His 274, His 320, and Asp 375. First, <scene name='Daniel_Eddelman_Sandbox_2/Asp_375/1'>Asp 375</scene> (a base) removes a proton from the methyl group of acetyl-CoA to form its enol. <scene name='Daniel_Eddelman_Sandbox_2/His_274/1'>His 274</scene> stabilizes the acetyl-CoA enolate by forming a hydrogen bond with the enolate oxygen. The enolate then nucleophilically attacks oxaloacetate’s carbonyl carbon, and His 320 donates a proton to oxaloacetate’s carbonyl group in a concerted step, forming citryl-CoA (which remains bound to the enzyme). Finally, citryl-CoA is hydrolyzed to citrate and CoA. | <scene name='Daniel_Eddelman_Sandbox_2/Cts_active_site/4'>active site</scene> of citrate synthase participate in acid-base catalysis: His 274, His 320, and Asp 375. First, <scene name='Daniel_Eddelman_Sandbox_2/Asp_375/1'>Asp 375</scene> (a base) removes a proton from the methyl group of acetyl-CoA to form its enol. <scene name='Daniel_Eddelman_Sandbox_2/His_274/1'>His 274</scene> stabilizes the acetyl-CoA enolate by forming a hydrogen bond with the enolate oxygen. The enolate then nucleophilically attacks oxaloacetate’s carbonyl carbon, and | ||
<scene name='Daniel_Eddelman_Sandbox_2/His_320/1'>His 320</scene> donates a proton to oxaloacetate’s carbonyl group in a concerted step, forming citryl-CoA (which remains bound to the enzyme). Finally, citryl-CoA is hydrolyzed to citrate and CoA. | |||
Revision as of 07:18, 28 February 2010
The Structure and Mechanism of Citrate SynthaseThe Structure and Mechanism of Citrate Synthase
Template:STRUCTURE 1ctsCitrate synthase is an enzyme active in the mitochondria, where it is responsible for catalyzing the first reaction of the citric acid cycle (Krebs Cycle): the condensation of acetyl-CoA and oxaloacetate to form citrate.
Structure: Citrate synthase is a single amino acid chain . Biologically, however, it exists as a . Each identical subunit consists of a large and a small domain, and is comprised almost entirely of α helices (making it an all α protein). In its free enzyme state, citrate synthase exists in “open” form, with its two domains forming a cleft containing the substrate (oxaloacetate) binding site (PDB: 1cts) [1]. When oxaloacetate binds, the smaller domain undergoes an 18° rotation, sealing the oxaloacetate binding site and resulting in the (PDB: 2cts). This conformational change not only prevents solvent from reaching the bound substrate, but also generates the acetyl-CoA binding site. This presence of “open” and “closed” forms results in citrate synthase having Ordered Sequential kinetic behavior.
Mechanism: The reaction mechanism for citrate synthase was proposed by James Remington. In this mechanism, three ionizable side chains in the of citrate synthase participate in acid-base catalysis: His 274, His 320, and Asp 375. First, (a base) removes a proton from the methyl group of acetyl-CoA to form its enol. stabilizes the acetyl-CoA enolate by forming a hydrogen bond with the enolate oxygen. The enolate then nucleophilically attacks oxaloacetate’s carbonyl carbon, and donates a proton to oxaloacetate’s carbonyl group in a concerted step, forming citryl-CoA (which remains bound to the enzyme). Finally, citryl-CoA is hydrolyzed to citrate and CoA.